In this courtroom sketch, Bradley Manning, second from left, sits as his attorney speaks during a military hearing in Fort Meade. Photograph: William Hennessy/AP

Eighteen months after his arrest in Iraq for allegedly orchestrating the largest leak of state secrets in American history, Bradley Manning has faced his military accusers for the first time, immediately turning the guns against them.

At the start of a preliminary hearing to establish whether the US soldier should face a full court martial for allegedly passing more than 250,000 US embassy cables to WikiLeaks, Manning issued a dramatic challenge to the investigating officer – the military equivalent to a judge – implying that the proceedings were biased and effectively rigged against him.

His lawyer, David Coombs, demanded that the investigating officer, Lt Colonel Paul Almanza, recuse himself from the case on the grounds that he works as a prosecutor for the US department of justice.

The DoJ is involved in a criminal pursuit of Julian Assange, founder of the whistleblowing website WikiLeaks, to which Manning is alleged to have transferred a huge trove of US embassy cables and other confidential materials.

At the end of a day of stuttering proceedings in which the court spent more time in recess than in active session, Almanza ruled that he would not remove himself from the case and that proceedings would continue on Saturday, Manning's 24th birthday. Almanza said that under the rules of military justice, a "reasonable person" would not conclude that he was incapable of conducting an impartial investigation into the charges against Manning.

But the defence counter-offensive managed to dominate the opening of the hearing and set a tone that could be played out in days to come. Coombs presented a vigorous case to the court that Manning was being denied a fair hearing, decrying the fact that while the prosecution had been allowed all 10 of its requested witnesses, only two of the 48 defence witnesses that Coombs had asked for were being allowed (other than the 10 shared with the prosecution).

"Two out of 48!" he exclaimed. "In a case in which the government has charged [Manning] with aiding the enemy, which carries the maximum sentence right now of death!"

Coombs also floated a theory (that could be fleshed out in future evidence) that the US government is engaging in an attempt to cajole Manning into turning state's witness against Assange. "If the department of justice got their way, they would get a plea in this case, and get my client to be named as one of the witnesses to go after Julian Assange and WikiLeaks," he said.

Manning made his first public appearance since 25 May 2010, when he was arrested at Forward Operating Base Hammer outside Baghdad. Dressed in military fatigues, wearing black-rimmed glasses and closely cropped hair, he looked strikingly small flanked by the looming figures of Coombs and his two military defence lawyers.

Manning spoke only to confirm to the investigating officer that he understood the charges against him. As the courtroom emptied at the end of the day's proceedings, a man shouted from the public gallery: "Bradley Manning, you're a hero"; the soldier made no reaction.

The soldier faces a maximum sentence of life in military custody with no chance of parole. The most serious charge against him – aiding the enemy – carries the death penalty, but the prosecutors have indicated they will not seek the ultimate punishment.

The full charge sheet was released for the first time. There are a total of 23 counts, including that Manning knowingly gave "intelligence to the enemy, through indirect means". The idea that WikiLeaks constitutes a conduit to an enemy of the US state will in itself be subject of much debate and legal argument.

A second charge accuses Manning of causing information to be published "having knowledge that intelligence published on the internet is accessible to the enemy".

He is also charged with passing information from a secure database containing more than 250,000 records belonging to the US government – a reference to the US embassy cables published by WikiLeaks through an international group of newspapers including the Guardian in November 2010.

A vigil in support of Manning was held outside the main gates of Fort Meade, situated in the state of Maryland.

Nobody knows how long the hearing, called an Article 32, could go on – judging from day one it could take several days. At the end of the process, Almanza will make a recommendation to the commander of the military district of Washington as to what should happen next.

The commander will have the final say over Manning's fate. He could refer the soldier to a general court martial for a full trial carrying the maximum sentence, opt for a lesser form of court martial, or dismiss some or all of the charges.